The present invention relates to methods and arrangements for reading bar coded information in the form of intervals of predetermined sizes between elements or bars.
One example of such coding is that currently used by banks to record identifying information on individual documents (such as checks) which are provided to customers to allow them to perform predetermined transactions. The identifying information may comprise the code numbers of the bank, the branch, the account of the drawer, and the type of document. It is on the basis of this particular example that the invention is described and illustrated herein. It will, however, be appreciated that the invention is not in any sense restricted to this particular example.
A plurality of codes have been adopted by businesses handling money. In Europe, and particularly in the Latin countries, the code most widely employed is that known by the abbreviation CMC 7 (meaning "Coded Magnetic Characters comprising 7 elements"). This is a method of representing alphabetical and numerical information which has the advantage of being readable both by a machine and by an untrained human. In effect, the way in which each group of seven elements, which are arranged vertically, parallel to one another, appears on the document suggests the shape of the character in question. In order to allow each character to be recognized by a machine, the seven elements are differently separated from one another by predefined short or long intervals. The numeral "0" for example is defined by, reading from left to right, a sequence of two short intervals, two long intervals and two short intervals. The numeral "1" is defined by one long interval, three short intervals, one long interval and one short interval; the letter A by one short interval, one long interval and four short intervals; the letter D by one long interval, two short intervals, two long intervals and one short interval; and the letter Z by two short intervals, two long intervals and two short intervals.
In addition, each character is separated from each adjacent character by an "inter character" interval which is longer than the other two intervals. The inter-character interval may also be termed a very long interval. The CMC 7 code requires the short, long and very long intervals to be of the following respective sizes: 300, 500 and 950 micrometers.
A more detailed description of the CMC 7 code, its manner of use, and reading devices therefor is found in the following U.S. Patents, the entire disclosures of which are hereby incorporated by reference: U.S. Pat. Nos. 3,044,696--Feissel; 3,286,233--Lesueur; and 3,309,667--Feissel et al.
The information is read by magnetic readers. Accordingly, the elements (also called bars) are printed on the document in an ink which can be permanently magnetized, and the reading heads are preceded by a magnetizing device designed to magnetically saturate the elements.
At the present time decoding is performed using one of the following two prior art principles or methods:
The first reading method comprises causing the document to move past only one reading head. The reading head responds to the magnetic transitions which the leading and trailing edges of each element represent in its air-gap by generating positive and negative pulses in the respective cases. The machine then uses as a reference only a single polarity of pulse to allow it to distinguish between the long and short intervals on the basis of the times at which pulses having this polarity appear. The physical intervals between bars are thereby converted by the machine into intervals of time. It will be apparent that detection can only be reliable if the speed of movement of the document relative to the reading head is stable.
In the second method of reading, the short and long intervals between elements are distinguished by spacing two reading heads apart in the direction of movement of the documents by a distance equal to one of the intervals. If the interval selected for reading head spacing is the short one, the two signals emitted by the two heads correspond when a short interval on the document is between them and they fail to correspond in the case of a long interval. If the interval selected for reading head spacing is the long one, the signals are just the opposite.
This second method of reading has the advantage over the first of dispensing with the measurement of the intervals in terms of time and providing instead physical measurement of the intervals. A detailed description of such readers will be found in a French patent application entitled "Arrangement for Reading Data" which was filed in France as No. 74-35418 on Oct. 22, 1974 by the societe anonyme styled "Compagnie Industrielle des Telecommunications CIT-ALCATEL", a company domiciled in France, and which is now French Patent No. 2,289,010. In addition, a reader implementing this second method is described below with particular reference to FIGS. 1 and 2.
A variation on the two-head reader is a three-head reader disclosed in a commonly-assigned copending U.S. patent application Ser. No. 936,693, filed Aug. 25, 1978 by Daniel Maussion, and entitled "BAR CODE READING DEVICE", the entire disclosure of which is hereby incorporated by reference. Briefly, in the Maussion bar code reading device a read head includes three element detectors physically spaced such that the first and second detectors are separated by a distance equal to a short interval and the first and third detectors are separated by a distance equal to a long interval. Suitable circuitry connected to the detectors identifies a short interval when the first and second detectors simultaneously detect character elements, and identifies a long interval when the first and third detectors simultaneously detect character elements.
This second method of detection or reading by comparing the degree of simultaneity with which the signals appear is theoretically independent of the speed of movement of the document past the two reading heads. However, if, like those described in the case of the first method of reading set forth above, the magnetic readers are sensitive to transitions in the magnetic induction field, the detected signal is proportional to the derivative of the magnetic induction field with respect to time (dB/dt), that is to say proportional to the relative speed of movement of the document in the case of magnetic transitions which are otherwise equal. To obtain correct signals from readers of this kind, it is therefore necessary to have relatively high speeds of movement. In this case, the second method of detection has only the advantage that the speeds do not have to be stabilized.
With the object of making detection genuinely independent of the speed of movement of the documents relative to the readers, readers have been produced using magneto-resistors. The latter are electrical resistors which are deposited on a substrate of insulating material in the form of thin films or layers of very small thickness (from a few hundred Angstroms to a few microns) whose resistance R varies when they are subjected to a magnetic field. In such a reader, a measuring magneto-resistor R is connected to the terminals of a current generator I. When an element or bar, duly magnetized, appears in front of this magneto-resistor, the magnetic field of the bar causes a change in resistance .DELTA.R in the resistor proportional to the strength of the magnetic field. This change is converted by the current generator into a variation in voltage .DELTA.V=(I) (.DELTA.R). The ratio .DELTA.R/R is termed the coefficient of magneto-resistance. Detailed descriptions of magnetic readers of this kind will be found in the following commonly-assigned U.S. patent applications, the entire disclosures of which are hereby incorporated by reference: Ser. No. 899,383, filed Apr. 24, 1978 by Jean-Pierre Lazzari and Michel Helle, and entitled "Magnetic Transducer Device for Detecting Coded Magnetic Information and Method of Producing Said Device"; and Ser. No. 899,217, filed Apr. 24, 1978 by Jean-Pierre Lazzari and entitled "Device for Reading Magnetic Information".
It will be seen from this brief description of magneto-resistive readers that they are not sensitive to changes in magnetic field but only react to the strength of this field, with the result that reading is completely independent of the speed of movement of the documents read.
To sum up, a reader having twin magneto-resistive heads differentiates between long and short intervals irrespective of the speed of reading. In operation, the machine counts six intervals and determines the relevant character and then counts another six intervals to determine the next character, thus repeating the count iteratively.
Although in theory this reader appears to be entirely satisfactory as regards standard of performance and reliability in reconstituting data, in practice it has major disadvantages.
One of its disadvantages lies in the production of the electronic circuits which are used to differentiate the short intervals from the long intervals. In effect, the operative edges of the signals from the two reading heads should theoretically be simultaneous when an interval between bars corresponds to that which separates the two heads. In actual fact, one of these two signals may arrive before or after the other because of slight differences in the position of the bars relative to one another, these slight differences being inherent in the devices for printing the bars and in the carrier or substrate selected. Therefore, to make a comparison between the two detected signals, it is necessary to delay one of the two signals so that in all possible cases it will always be delayed with respect to the other signal. This makes it necesary to have more extensive processing electronics than are required with the above-mentioned first method of reading.
However, the basic cause of the major disadvantages of twin-head readers produced hitherto is that they detect only two intervals (long and short) out of the three which exist, and these disadvantages become apparent when documents are being read which contain inking faults inherent in the coding techniques (a lack of ink or, conversely, the presence of ink splodges).
In effect, it was seen above that, in detection by the second method, the fact of the signals read being simultaneous confirms the existence of a given interval (short for example) if this is the interval by which the two heads are separated; while if the two detected signals are not simultaneous, the other interval (long in this example) is assumed to be present. However, in reality, the fact of non-simultaneity only proves that the interval is not short, i.e., it may be long or very long. This being the case, it will be appreciated how many uncontrollable confusions there may be when, because of an inking fault which is very common in CMC 7 coding, there is a wrong interval in six intervals counted, which may then falsify the whole of the reading of the corresponding document.
By way of example, let it be assumed that the reader has its two reading heads separated by a short interval and that it is presented with an actual character D (the code of which is as defined above: one long interval, two short intervals, two long intervals and one short interval) having two inking faults as follows: a lack of ink in the first bar (that furthest to the left of the document) and the presence of an ink splodge in front of the last bar (this splodge simulating the existence of another bar marking a short interval). If it is also assumed that the reading takes place from the right of the document to the left, the machine will first of all read the false short interval simulated by the ink splodge and then a genuine short interval, two long intervals, and two short intervals, these six intervals representing the character Z defined above. The lack of ink in the first bar, which is the last one read, will not reflect the error in the rest of the reading. However, when it is present, the machine will consider the long interval which separates the first bar from the second bar as a very long (inter-character) interval and the very long interval which follows as the first long interval of a second character, which will cause the next item of information to be wrongly interpreted.
The first method of reading, by virtue of its absolute detection of the intervals, is capable of recognizing all the kinds of interval, provided that the speed of movement of the document is very stable, and thus of indicating to the operator the existence of a fault in the printing of a character. However, the production of such readers and the restrictions which they impose in operation constitute a serious handicap, since, in addition to the fact that the magnitude of the read-out signal is a function of the speed of movement of the document relative to the reading head, the presence of air-gaps means that the head is very sensitive to the distance separating it from the document, the maximum distance generally permitted being equal to the size of the air-gap (in the order of a few tens of microns).